Joining between metallic or thermoplastic and composite components is currently approached in a number of ways, each with its own limitations.
The use of fasteners is commonplace but tends to result in de-lamination around fastener holes. Fastener holes are often difficult to drill in composites and significant reinforcement around fastener holes may be required, leading to increased weight. Fastened joints tend to be particularly weak in the pull-through direction (that is, the direction of axial load through the fastener). As such, fastened joints are not well suited to many aerospace applications.
Adhesive bonds are an increasingly common means of joining metallic components to composite laminates, however these perform poorly in peel, tension and cleavage, and tend to fail with little or no warning. Their weakness in peel and in tension makes bonded joints similarly limited in their application within conventional aerospace structures. Any mitigation for the poor performance in peel or tension tends to result in large bond surface areas, with the associated weight penalties that go with this.
WO 2004/028731 A1 describes a method by which surface features are generated by using a ‘power-beam’ such as an electron beam, in order to ‘flick-up’ surface material on a metallic component to sculpt protruding features that are intended to increase bond surface area and improve bond strength when incorporated into the matrix of a co-cured laminate.
WO 2008/110835 A1 describes a method by which surface features are “grown” on a bond surface of a component in a series of layers by an additive fabrication process. The profile and shape of the surface features can be readily controlled so as to optimise the performance of the joint, particularly in tension and peel. Each surface feature may have a pointed tip such that the surface features may easily be embedded into series of laminate plies draped successively over the bond surface. The surface features may by asymmetrical so as to improve properties of the joint in a particular load direction.
However, the above methods suffer the problem that at least some of the surface features may tear through the laminate plies during formation of the joint, leading to a reduction in the strength of the completed joint. This problem is especially acute where the component is a corner bracket, or the like, such that the laminate plies need to be draped over a corner.